1. Field of the Invention
This invention generally concerns starters for large rotating equipment. More specifically, it concerns a variable speed fluid drive (torque converter) starter for a string of rotating equipment that includes multiple high load compressors. The fluid drive starter is an alternative device for starting either a large motor or turbine to drive a compressor(s) or other rotating machinery based load.
2. Discussion of Background Information
Combinations of high speed, high power rotating equipment, known as strings, require starting mechanisms to initiate operation. A typical string in a facility may have a gas turbine or motor driver connected to a compressor, multiplicity of compressors, generator, or another rotating machinery based load. A starter mechanism such as a low power starting motor may also be connected to the string. FIG. 1 shows an example of a string with a gas turbine mechanical drive and compression load with a variable frequency drive (VFD) 1 with starter motor 2. The VFD 1 is an electric device that inverts fixed AC line input voltage to DC and converts DC voltage to user-defined output AC. The VFD produces a user selectable frequency output, thereby providing variable speed for the motor. As a result, large inertial loads are started with limited (controlled) in-rush current as opposed to across-the-line starts in synchronous motors with damper bars which may draw up to 6 times (load dependent) the motor rated current (continuous duty current). FIG. 2 schematically illustrates an across-the-line starter motor (S/M) 3 with a variable speed hydraulic clutch (HC) 4. The across-the-line starter motor with HC is another mechanism for starting large inertial loads. The HC operates as a mechanical VFD. The across-the-line starter motor is started with the HC disengaged. Once the motor is at full speed, the HC is engaged providing variable speed (zero to full speed) and necessary torque to bring the gas turbine and other connected load(s) to full speed. Once the gas turbine achieves sufficient speed to produce power, the hydraulic clutch is disengaged and the starter motor is electrically removed from service.
Starting a string may be achieved under one of two primary conditions. The first condition is depressurized start, and the second is pressurized start. A depressurized start initiates at a low settle-out pressure within the compressor(s). For a depressurized start, the working gas is removed from the compressor(s). This gas must be replaced. The method for replacement is known as gas make-up. Gas make-up may require extra facility hardware (valves, piping, transmitters, flares, gas reclamation, and associated controls) and is a time consuming effort. Due to a lengthy time requirement and extra facility cost to make-up gas in a depressurized start, pressurized start is an attractive alternative. A pressurized start initiates with a high settle-out pressure within the compressor(s) when compared to depressurized start. A pressurized start removes necessary hardware associated with gas make-up in depressurized starts. However, a pressurized start requires additional starting power due to higher starting torque.
There are two types of turbines employed in mechanical drive service, dual-shaft and single-shaft. Dual-shaft gas turbines produce compressed air for the combustion process with a compressor driving a short stage turbine on a single shaft. The remaining thermodynamic power in the form of pressure, temperature, and mass flow is routed directly into the coaxial power turbine on a second shaft. The advantage of a dual-shaft gas turbine is the ability to produce significant power (torque) across the turbine's speed range. However, as compressor string power requirements have increased, the demand for larger power gas turbines has also increased. To meet these power requirements, users have adapted single shaft gas turbine technology (traditionally used in power generation) for mechanical drive service. In addition to gas turbine mechanical drives, motors are also used as compressor prime movers. These large motors are conventionally driven by VFDs.
For large motors under a pressurized start, across the line starting is not possible. Variable frequency drives as in FIG. 1 are a necessity. In a typical example, four compressors in a string are required to be started. Each compressor requires 30 MW at 3,000 rpm at operating design point. These compressors require a total of 120 MW to operate. In this example, it is assumed that the pressurized start requires all 120 MW to start the string. With present technology (VFD), it is costly to start with full pressure. The string would require a depressurized start and/or compressor suction throttling resulting in starting power at full speed to be less than 75 MW. A turbine drive capable of meeting the 120 MW power requirements is necessary. Turbines of this size are incapable of producing significant power at speeds below 97% of full rated speed and are insufficient for starting. The string requires a starter/helper motor and VFD to provide the necessary starting power for the string. After the string achieves full speed at motor maximum or near maximum power, the turbine will apply the remaining power to achieve 120 MW. At this point, starting power from the motor is no longer required.
However, a VFD is a large capital expense and may contribute up to 70% of the cost in a motor/VFD package. A mechanical alternative to starting a string uses a variable speed fluid drive between the turbine or motor and the compressors. A variable speed fluid drive (VSFD) is a constant speed input, variable speed output, device that transmits power (torque) from the input shaft to the output shaft via a fluid (hydraulic) coupling. A more common name for a variable speed fluid drive is a compressor starting torque converter (CSTC). The term CSTC will be used throughout this application specifically and interchangeably to refer to variable speed fluid drive torque converters. U.S. Pat. No. 6,463,740, issued Oct. 15, 2002 to Schmidt, et al. (“Schmidt”) is an example of a known CSTC.
FIG. 3 depicts a known drive having a prime mover (see, for example, Schmidt) which may be a gas turbine or electric motor, with a CSTC 5. The CSTC eliminates the need for a VFD in a motor driven string and eliminates the need for a starter/helper motor and VFD in a gas turbine driven string. However, a starter motor with a hydraulic clutch is required in either case to start the prime mover. While FIG. 3 specifically shows two compressors, Schmidt discloses the use of at least one compressor.
Power to speed performance of the CSTC is limited. In the related application of the current inventors, U.S. Provisional No. 60/779,680 referred to above, the power to speed dilemma is addressed using a gear train to enable the CSTC to operate at a lower speed which can be increased to match the speed of the turbine or motor. But, certain applications having multiple compressors require more power at a given speed than a single CSTC can support. In the example discussed above, current VSFD technology at 3,000 rpm can only start up to 30 MW. After the synchronous speed lock up mechanism is engaged, it may transfer up to 130 MW. With these power limits, a single CSTC cannot start the multiple compressors in the string.
In a depressurized start, where the loads in a compressor string are reduced, a CSTC may be capable of performing a start. However, for pressurized starts in high-load, multiple compressor strings the power of a single CSTC is often insufficient to start multiple compressors. The present invention seeks to mitigate this shortcoming.